Manual: Typo in subset axiom title (#186)

Co-authored-by: Sankalp Gambhir <sankalp.gambhir47@gmail.com>
Co-authored-by: SimonGuilloud <simon.guilloud@bluewin.ch>

Reference Manual/theory.tex

@@ -43,8 +43,8 @@ This is however not a convenient definition. In practice, all infinite axiomatiz
   \begin{axz}[extensionality]\label{axz:extensionality}
     $\forall x, y. (\forall z. z \in x \iff z \in y) \iff (x = y)$
   \end{axz}
-  \begin{axz}[extensionality]\label{axz:subset}
-    $\forall x, y. x\subset y \iff \forall z. z \in x iff z \in y$
+  \begin{axz}[subset]\label{axz:subset}
+    $\forall x, y. x\subset y \iff \forall z. z \in x \iff z \in y$
   \end{axz}
   \begin{axz}[pair]\label{axz:pair}
     $\forall x, y, z. (z \in (x, y)) \iff ((x \in y) \lor (y \in z))$